Since the 1970's, port-fuel injected engines have utilized three-way catalysts and closed-loop engine controls in order to seek to minimize NOx, CO, and unburned hydrocarbon emissions. This strategy has proven to be particularly effective during normal operation in which the engine and exhaust components have reached sufficient temperatures. However, in order to achieve desirable conversion efficiencies of NOx, CO, and unburned hydrocarbons, the three-way catalyst must be above its inherent catalyst light-off temperature.
In addition, the engine must be at sufficient temperature to allow for vaporization of liquid fuel as it impinges upon intake components, such as port walls and/or the back of valves. The effectiveness of this process is important in that it provides a proper degree of control over the stoichiometry of the fuel/air mixture and, thus, is coupled to idle quality and the performance of the three-way catalyst, and it ensures that the fuel supplied to the engine is burned during combustion and, thus, eliminates the need for over-fueling to compensate for liquid fuel that does not vaporize sufficiently and/or collects on intake components.
In order for combustion to be chemically complete, the fuel-air mixture must be vaporized to a stoichiometric gas-phase mixture. A stoichiometric combustible mixture contains the exact quantities of air (oxygen) and fuel required for complete combustion. For gasoline, this air-to-fuel ratio is about 14.7:1 by weight. A fuel-air mixture that is not completely vaporized, and/or contains more than a stoichiometric amount of fuel, results in incomplete combustion and reduced thermal efficiency. The products of an ideal combustion process are water (H2O) and carbon dioxide (CO2). If combustion is incomplete, some carbon is not fully oxidized, yielding carbon monoxide (CO) and unburned hydrocarbons (HC).
Under cold-start and warm-up conditions, the processes used to reduce exhaust emissions and deliver high quality fuel vapor break down due to relatively cool temperatures. In particular, the effectiveness of three-way catalysts is not significant below approximately 250° C. and, consequently, a large fraction of unburned hydrocarbons pass unconverted to the environment. Under these conditions, the increase in hydrocarbon emissions is exacerbated by over-fueling required during cold-start and warm-up. That is, since fuel is not readily vaporized through impingement on cold intake manifold components, over-fueling is necessary to create combustible mixtures for engine starting and acceptable idle quality.
The mandates to reduce air pollution worldwide have resulted in attempts to compensate for combustion inefficiencies with a multiplicity of fuel system and engine modifications. As evidenced by the prior art relating to fuel preparation and delivery systems, much effort has been directed to reducing liquid fuel droplet size, increasing system turbulence and providing sufficient heat to vaporize fuels to permit more complete combustion.
However, inefficient fuel preparation at lower engine temperatures remains a problem which results in higher emissions, requiring after-treatment and complex control strategies. Such control strategies can include exhaust gas recirculation, variable valve timing, retarded ignition timing, reduced compression ratios, the use of catalytic converters and air injection to oxidize unburned hydrocarbons and produce an exothermic reaction benefiting catalytic converter light-off.
As indicated, over-fueling the engine during cold-start and warm-up is a significant source of unburned hydrocarbon emissions in conventional engines. It has been estimated that as much as 80 percent of the total hydrocarbon emissions produced by a typical, modern port fuel injected (PFI) gasoline engine passenger car occurs during the cold-start and warm-up period, in which the engine is over-fueled and the catalytic converter is essentially inactive.
Given the relatively large proportion of unburned hydrocarbons emitted during startup, this aspect of passenger car engine operation has been the focus of significant technology development efforts. Furthermore, as increasingly stringent emissions standards are enacted into legislation and consumers remain sensitive to pricing and performance, these development efforts will continue to be paramount. Such efforts to reduce start-up emissions from conventional engines generally fall into two categories: 1) reducing the warm-up time for three-way catalyst systems and 2) improving techniques for fuel vaporization. Efforts to reduce the warm-up time for three-way catalysts to date have included: retarding the ignition timing to elevate the exhaust temperature; opening the exhaust valves prematurely; electrically heating the catalyst; burner or flame heating the catalyst; and catalytically heating the catalyst. As a whole, these efforts are costly and do not address HC emissions during and immediately after cold start.
A variety of techniques have been proposed to address the issue of fuel vaporization. U.S. patents proposing fuel vaporization techniques include U.S. Pat. No. 5,195,477 issued to Hudson, Jr. et al, U.S. Pat. No. 5,331,937 issued to Clarke, U.S. Pat. No. 4,886,032 issued to Asmus, U.S. Pat. No. 4,955,351 issued to Lewis et al., U.S. Pat. No. 4,458,655 issued to Oza, U.S. Pat. No. 6,189,518 issued to Cooke, U.S. Pat. No. 5,482,023 issued to Hunt, U.S. Pat. No. 6,109,247 issued to Hunt, U.S. Pat. No. 6,067,970 issued to Awarzamani et al., U.S. Pat. No. 5,947,091 issued to Krohn et al., U.S. Pat. No. 5,758,826 issued to Nines, U.S. Pat. No. 5,836,289 issued to Thring, and U.S. Pat. No. 5,813,388 issued to Cikanek, Jr. et al.
Other fuel delivery devices proposed include U.S. Pat. No. 3,716,416, which discloses a fuel-metering device for use in a fuel cell system. The fuel cell system is intended to be self-regulating, producing power at a predetermined level. The proposed fuel metering system includes a capillary flow control device for throttling the fuel flow in response to the power output of the fuel cell, rather than to provide improved fuel preparation for subsequent combustion. Instead, the fuel is intended to be fed to a fuel reformer for conversion to H2 and then fed to a fuel cell. In a preferred embodiment, the capillary tubes are made of metal and the capillary itself is used as a resistor, which is in electrical contact with the power output of the fuel cell. Because the flow resistance of a vapor is greater than that of a liquid, the flow is throttled as the power output increases. The fuels suggested for use include any fluid that is easily transformed from a liquid to a vapor phase by applying heat and flows freely through a capillary. Vaporization appears to be achieved in the manner that vapor lock occurs in automotive engines.
U.S. Pat. No. 6,276,347 proposes a supercritical or near-supercritical atomizer and method for achieving atomization or vaporization of a liquid. The supercritical atomizer of U.S. Pat. No. 6,276,347 is said to enable the use of heavy fuels to fire small, light weight, low compression ratio, spark-ignition piston engines that typically burn gasoline. The atomizer is intended to create a spray of fine droplets from liquid, or liquid-like fuels, by moving the fuels toward their supercritical temperature and releasing the fuels into a region of lower pressure on the gas stability field in the phase diagram associated with the fuels, causing a fine atomization or vaporization of the fuel. Utility is disclosed for applications such as combustion engines, scientific equipment, chemical processing, waste disposal control, cleaning, etching, insect control, surface modification, humidification and vaporization.
To minimize decomposition, U.S. Pat. No. 6,276,347 proposes keeping the fuel below the supercritical temperature until passing the distal end of a restrictor for atomization. For certain applications, heating just the tip of the restrictor is desired to minimize the potential for chemical reactions or precipitations. This is said to reduce problems associated with impurities, reactants or materials in the fuel stream which otherwise tend to be driven out of solution, clogging lines and filters. Working at or near supercritical pressure suggests that the fuel supply system operate in the range of 300 to 800 psig. While the use of supercritical pressures and temperatures might reduce clogging of the atomizer, it appears to require the use of a relatively more expensive fuel pump, as well as fuel lines, fittings and the like that are capable of operating at these elevated pressures.